杏吧原创

NEXT July, a trio of spacecraft will begin their journeys to Mars. The US, Europe and China are all taking advantage of a favourable planetary alignment to launch rovers to the Red Planet and search for signs of current and past life. But what happens if a stray Earth microbe hitches a ride?

That is where the principle of planetary protection comes in. The idea is to make sure that microorganisms from Earth don鈥檛 end up on other worlds 鈥� and vice versa. This is both to avoid killing off any extant life that might be on another planet and to make sure that we can still study those worlds in a relatively pristine state.

鈥淚t鈥檚 really easy to find life on Mars or anywhere else,鈥� says Catharine Conley, who was NASA鈥檚 planetary protection officer from 2006 until 2018. 鈥淛ust bring it with you.鈥�

In an effort to avoid this, international planetary protection rules have been in place since the 1950s, but the rise of the private space-flight industry has prompted a rethink. If we end up in a future where SpaceX regularly lands humans on Mars, we want to avoid contamination as much as possible. At the same time, onerous regulations could restrict the growing industry.

That is why, in mid-2019, NASA convened an independent board of researchers and space-flight firms to review its planetary protection policies and suggest ways to modernise the process. The board presented its final report to the international Committee on Space Research (COSPAR) last week, with a view to the protocols being adopted worldwide. SpaceX took part in the review, but didn鈥檛 respond to New 杏吧原创鈥榮 request for comment.

Some of the existing rules are a bit of a mess, says Lisa Pratt, NASA鈥檚 current planetary protection officer. 鈥淵ou have requirements that are impossible to verify or measure,鈥� she says. 鈥淪ome things are written in terms of the probability of introducing a single viable organism into a body of water, and we have no reliable or precise method to determine how many viable spores there are on a spacecraft.鈥�

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The number of Mars rovers due to launch in 2020

The report doesn鈥檛 suggest how NASA鈥檚 protocols should change to deal with this, but it does suggest that they might not be nuanced enough. For example, when a spacecraft is en route to its destination, the cold and radiation of space will kill many onboard microorganisms. That might mean we could let spacecraft launch with more of these sorts of microorganisms than we do currently, the report says.

It also calls for a smarter consideration of a spacecraft鈥檚 destination, because different areas on the same world have different potential for biology. This principle is already in use today 鈥� for example, rovers are free to visit most of Mars, but areas that might have liquid water are deemed by COSPAR to be 鈥渟pecial regions鈥� that nobody can visit for fear of contamination.

鈥淲hen planetary protection got started, Mars was a point of light in the sky, and now it鈥檚 a real world with countless different places on it,鈥� says Alan Stern at the Southwest Research Institute in Texas, chair of the review board. 鈥淲e don鈥檛 have to protect them all equally.鈥� (See diagram.)

The board recommends that this system of 鈥渟pecial regions鈥� be expanded to other worlds besides Mars. For example, right now missions to land on Europa or Enceladus, two moons with water oceans beneath their icy shells, must have essentially no chance of bringing microorganisms to the surface. The report says that might be unnecessarily strict.

鈥淥ne would think that Earth life would be easily distinguishable from life that came about there,鈥� because of the huge differences in the conditions under which it evolved, says Amanda Hendrix at the Planetary Science Institute in Arizona, who was also a member of the review board. Even if we did contaminate these worlds, the chance that it would ruin future science seems small, she says.

To quantify this, the report calls for NASA to study how microbes could survive, reproduce and move around each ocean world. Understanding whether microbes could be transported by winds, ice flowing and melting, and other climate patterns will be key to deciding how different areas need to be protected, says Pratt.

Those transport mechanisms could be a sticking point for planetary protection when it comes to sending humans to Mars. We understand more about the Red Planet than we ever have before, but there are constant new discoveries about the surface, the subsurface and water on Mars. Sending humans there, which is one of NASA鈥檚 explicit goals, will require breaking just about every planetary protection guideline there is.

鈥淭he minute we send humans to Mars, and they start pooing on Mars, forget about baking the spacecraft in cleanrooms to get rid of the microbes,鈥� says Paul Byrne, a planetary scientist at North Carolina State University. Humans are overflowing with microbes of all sorts and it is inevitable that some will end up on the Martian surface, even though people will be wearing spacesuits.

We can鈥檛 be sure that those microbes won鈥檛 spread around the Red Planet and obliterate any signs of past or present life there, but Stern says that the Martian environment is poisonous enough to Earth life that we shouldn鈥檛 worry too much. 鈥淗aving humans on the surface will allow us to do science that we cannot do efficiently with robots,鈥� he says. 鈥淚t鈥檚 a little bit of downside and a lot of upside.鈥�

Of course, NASA isn鈥檛 the only agency with its sights set on the moon, Mars and maybe even other destinations in the solar system. The whole thing is made more complicated 鈥� and more urgent 鈥� by the increasing accessibility of space flight. Many countries with smaller space programmes are aiming for solar system exploration, as are private companies like SpaceX, and NASA鈥檚 policies don鈥檛 necessarily apply to them.

鈥淚t鈥檚 sort of an honour system,鈥� says Hendrix. There are some international requirements through COSPAR and the Outer Space Treaty of 1967, but those requirements tend to be more lenient than NASA鈥檚.

Expensive burden

One rationale for the report was to make it easier for industry to comply with planetary protection protocols. Adhering to strict rules could make things prohibitively expensive, especially for relatively small missions.

鈥淲e want to take planetary protection seriously, yet we don鈥檛 want to burden missions more than necessary,鈥� says Hendrix. 鈥淲e want to encourage exploration 鈥� that鈥檚 what NASA is all about. So if it鈥檚 too burdensome, we鈥檙e not doing our job.鈥�

Conley takes a stricter approach. She describes the idea of loosening planetary protection constraints as 鈥減enny wise, pound foolish鈥� and says that it risks harming future exploration for the sake of speeding it up now.

鈥淵ou don鈥檛 know that you鈥檝e loosened them too much until you make a mistake,鈥� says Conley.

鈥淲e鈥檙e going to go out and explore, so we need to make sure that we don鈥檛 do something truly stupid鈥�

Such a mistake was narrowly avoided in the search for life-friendly regions on the Martian surface, she says. Restrictions were relatively loose, because it seemed there was little chance of finding life on Mars, and they were tightened in 2003 after Mars Global Surveyor saw apparent signs of flowing liquid water.

Unless everybody with space flight capabilities buys in to planetary protection, it won鈥檛 matter 鈥� one dirty spacecraft could potentially destroy any chance of finding life on an entire world.

As such, further study of these alien worlds will be crucial, says Byrne. It could be time to loosen protections on the moon and Mars 鈥� explorers are raring to go 鈥� but it might be better to wait to decide how we treat complex bodies like Europa and Titan until we understand them better.

鈥淲e鈥檙e going to go out and explore, so we need a framework in place to make sure that we don鈥檛 do something truly stupid,鈥� says Byrne. Contamination will happen, but we protect the future of space exploration by doing our best to minimise it.